Effect of high oxygen partial pressure heat treatment on the superconducting properties of Bi2Sr2CaCu2Ox/Ag tapes

We investigated the effect of oxygen partial pressure pO2 during melt-and-solidification heat treatment on the superconducting properties of grain oriented Bi-2212 thick films. Tc and c-axis parameter decreased with increasing pO2, indicating the samples were in the over-doped state. The heat treatment temperature which gave highest Jc increased with increasing pO2. This was explained by the increase of the solidification temperature of Bi-2212 with pO2. Both the resistive Jc and intergrain Jc estimated from the dc magnetization increased with increasing pO2 when the samples were melt-solidified under an optimum condition for each pO2. Amplitude dependence of ac susceptibility suggested that the coupling of Bi-2212 grains was improved with increasing pO2. Irreversibility field Birr below ∼60 K was also enhanced with increasing pO2. Low temperature annealing under high pO2 after the melt-solidification also increased the oxygen content in Bi-2212 and hence Birr. However, the annealing did not change the amplitude dependence of ac susceptibility, and transport Jc at 4.2 K was almost independent of the annealing. The results indicate that the increase of Jc at 4.2 K with increasing pO2 during melt-solidification is primarily attributed to the improvement of the grain couplings.